The invention relates to a fuel injection system for an internal-combustion engine particularly a diesel engine, which contains a number of fuel injectors for injecting fuel into the combustion spaces of the internal-combustion engine and a high-pressure pump which supplies the fuel injectors with fuel by way of a common inflow pipe and high-pressure lines leading to the individual fuel injectors, as well as high-pressure storage devices which have a defined fuel storage volume and are provided in the high-pressure lines leading to the fuel injectors.
In the case of internal-combustion engines, particularly large-volume diesel engines, fuel injection systems are increasingly used in which fuel is supplied from a fuel supply by means of a high-pressure pump by way of a high-pressure line into a high-pressure storage device. From the high-pressure storage device acting as an oil-elastic storage device, the fuel is supplied by way of additional high-pressure lines to fuel injectors which inject the fuel during injection operations into the combustion spaces of the internal-combustion engine. So far, the high-pressure storage device has predominantly been provided in the form of a so-called common rail which is a tube-shaped elongated element from which the high-pressure lines branch off which supply the individual fuel injectors. Furthermore, fuel injection systems are known in the case of which, in addition to or instead of such a common high-pressure storage device in the form of a common rail, high-pressure storage devices are provided as individual storage devices separately for each fuel injector.
Fuel injection systems, in the case of which, in addition to a common high-pressure storage device, one individual storage device respectively is provided separately for each fuel injector are known from German Patent Documents 43 41 543 A1 and DE 43 41 546 A1. In the case of the fuel injection system known from the former document, a parallel switching device consisting of a return valve, which is interconnected in the fuel conveying direction, and of a throttle is provided in the high-pressure line leading from the common high-pressure storage device to the high-pressure storage device provided separately for each fuel injector, the system prevents an uncontrolled flowing-back of fuel from the separate high-pressure storage device into the common high-pressure storage device and prevents an influencing of the pressure in the separate pressure storage spaces of the other fuel injectors, while to the return valve permits a rapid refilling of the separate high-pressure storage devices from the common high-pressure storage device. Such measures for avoiding the mutual influencing of the pressure in the separate high-pressure storage devices are not provided in the case of the fuel injection system known from the second document.
Furthermore, a fuel injection system is known from German Patent Document DE 43 44 190 A1, in which fuel is fed under a high pressure to high-pressure storage devices separately provided for each fuel injector from a fuel supply by way of high-pressure lines by means of a high-pressure pump. A shut-off valve is connected into each high-pressure line connecting a fuel injector with the assigned high-pressure storage device, which shut-off valve has the purpose of limiting the fuel quantity flowing through during a pressure interval which is characterized by the pressure drop occurring with the injection operation or in the case of a leakage. However, measures are not indicated by means of which the mutual influencing of the pressure in the individual high-pressure storage devices is to be avoided.
It is an object of the invention to provide an improved fuel injection system for an internal-combustion engine.
According to the invention, a fuel injection system for an internal-combustion engine, particularly a diesel engine, is provided which contains a number of fuel injectors for injecting fuel into the combustion spaces of the internal-combustion engine as well as a high-pressure pump which supplies fuel to the fuel injectors by way of a common inlet pipe and to high-pressure lines leading to the individual fuel injectors. In the high-pressure lines leading to the fuel injectors, high-pressure storage devices are provided which each have a defined fuel storage volume. According to the invention, it is provided that the fuel storage volume of each high-pressure storage device amounts to between 80 and 300 times, preferably between 120 and 200 times the maximal injection quantity per injection operation, and that the diameter D.sub.2 of the high-pressure lines leading from the common inlet pipe to the high-pressure storage devices is dimensioned such that the difference in the quantities injected by the fuel injectors assumes a minimum.
The invention is essentially based on the recognition that a variation of the diameter D.sub.2 of the high-pressure lines leading from the common inlet pipe to the high-pressure storage devices results already within a relatively small range in a significant change of the difference in the quantities injected by the individual fuel injectors and has a pronounced minimum in the range of the optimal diameter. By using this minimum, the fuel injection system can be designed such that the difference in the injected quantities is minimal and uniform ignition pressures can be achieved from one cylinder to the next. The minimum of the difference in the injected quantities indicates that the mutual influence of the individual pressure storage spaces also has a minimum.
A significant advantage of the fuel injection system according to the invention is the fact that the high-pressure lines supplying the individual high-pressure storage devices can have a small diameter and are therefore easy to bend and mount. It is another advantage that, for minimizing the mutual influencing of the individual high-pressure storage devices, no additional elements, such as return valves, are required. For example, because of the inertia of masses of the moved parts, valves have a delayed response behavior so that the propagation of pressure disturbances and the mutual influencing of the injectors cannot be prevented thereby. Since, in the case of the injection system according to the invention, the high-pressure lines supplying the individual high-pressure storage devices exercise a throttling effect on the fuel flowing through, a self-protection of the internal-combustion engine continues to exist against an overspeed because, in the event of the occurrence of such an overspeed, the high-pressure storage devices are being no longer completely filled.
It is preferably provided that the diameter D.sub.2 of the high-pressure lines leading from the common inlet pipe to the high-pressure storage devices meets the following requirement: EQU D.sub.2.apprxeq.((4.times.(V.sub.E +V.sub.L))/cg.sub.2.times..pi..times.T.sub.ASP)).sup.1/2
wherein
V.sub.E is the maximal injection volume per injection operation, PA1 V.sub.L is a control and leakage quantity per injection operation possibly occurring at the fuel injector, PA1 cg.sub.2 is a standard value for the fuel flow rate in the line, PA1 T.sub.ASP is the time duration for an operating cycle of the internal-combustion engine; wherein the values for V.sub.E, V.sub.L and T.sub.ASP are defined by the layout of the internal-combustion engine, and cg.sub.2 is to amount to between 5 and 50 m/s. In the case of long thin lines, the value for cg.sub.2 is between 5 and 25 m/s, and preferably between 7 and 9 m/s. In the case of short lines or throttle-type transitions, the value for cg.sub.2 is between 10 and 50 m/s, preferably between 35 and 45 m/s. As a result, the optimal value for the diameter D.sub.2 can be mathematically determined already during the layout of the internal-combustion engine. However, it is advantageous to determine by means of a variation of the diameter D.sub.2 about the thus obtained value the value for the diameter D.sub.2 of the high-pressure lines in practical tests, in the case of which diameter the difference in the quantities injected by the fuel injectors is in fact minimal, as explained above. PA1 V.sub.E is the maximal injection volume per injection operation, PA1 cg.sub.4 is the permissible maximal flow rate of the fuel in the high pressure line 4, and PA1 SD is the duration of the injection operation.
According to a further development of the invention, it is provided to determine the diameter D.sub.4 of the high-pressure lines leading from the high-pressure storage devices to the fuel injectors to be at least so large that the flow rate cg.sub.4 of the fuel in these high-pressure lines during the injection operation is no higher than 30 m/s, preferably no higher than 25 m/s.
Preferably, it is provided that the diameter D.sub.4 of the high-pressure lines leading from the high-pressure storage devices to the fuel injectors meets the following requirement: EQU D.sub.4.gtoreq.((4.times.V.sub.E)/(cg.sub.4.times..pi..times.SD)).sup.1/2
wherein
According to another advantageous further development of the invention, it is provided that the following applies to the diameters D.sub.1 of the common inlet pipe and D.sub.2 of the high-pressure lines leading from the common inlet pipe to the high-pressure storage devices: EQU D.sub.1.gtoreq.n.sub.R.sup.1/2.times.D.sub.2
wherein n.sub.R is the number of fuel injectors 5 connected to the common inlet pipe 1.
It is advantageously provided that the lengths of the high-pressure lines leading from the common inflow pipe to the high-pressure storage devices are identical. As a result, it is to be ensured that the mutual influencing of the individual high-pressure storage devices is the same for all fuel injectors. Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.